Over time, a semiconductor integrated circuit may become aged and experience higher resistances and lower drive capabilities in its transistors. These effects have become more pronounced as semiconductor integrated circuits have been scaled down.
To compensate for the aging effects, a frequency guard band has been used during testing of a new integrated circuit that was recently manufactured. That is, the integrated circuit is tested to operate with a clock rate at a percentage guard band (GB) above the rated frequency. The percentage GB is predetermined through experiments performed during process development. In this manner if the integrated circuit ages accordingly, it has a higher operating probability over time.
However, not all integrated circuits age in accordance with experiments performed during process development. There is still a small probability that new integrated circuits, with a guard banded frequency rating, will fail as they age. That is, these circuits may fail because they experience greater than expected aging.
A typical consumer integrated circuit may last seven years, at which point, one percent of their cumulative totals may have experienced catastrophic failure due to the unexpected degradation of their maximum operating frequencies, Fmax.
Moreover, some new integrated circuits are robust and can be operated at frequencies greater than the rated operating frequency without ever failing over time. In this case, guard banding the frequency of the integrated circuit to a rated operating frequency is inefficient, as lower than expected aging may occur. These new integrated circuits are never utilized to the extent of their capabilities. The experiments performed during process development to determine the percentage GB for guard banding do not actually reflect how an integrated circuit is used in a system.